The present invention concerns electromechanical filters of the type having at least one pole or maximum attenuation in the neighbourhood of the pass band. It relates to a filter structure of this type which is of simple mechanical construction, which permits industrial production and which ensures high quality of the transmission in the band to be transmitted.
Electromechanical filters consist essentially of a set of resonators, generally metal rods of cylindrical form, of which the dimensions are selected so that the said bars constitute circuits tuned to a preselected frequency in a chosen mode of vibration. These rods are interconnected by means of couplers, which generally consist of metal wires vibrating in a preselected mode and ensuring mechanical coupling between adjacent resonators. One of the resonators, called the input resonator, is associated with a transducer which changes an electrical input signal into a corresponding mechanical displacement in the mode of vibration chosen for the said rod; likewise, a second rod of the set, called the output resonator, is associated with a second transducer which effects the inverse conversion. The transducers are connected to the input and output terminals respectively of the filter. Such filters have been the subject of many developments which have been abundantly described in the technical press and patents, such as the following patent applications filed by the Assignee: Ser. No. 695,920 for "Electromechanical band-pass filter for high frequencies" dated 14th June, 1976 now U.S. Pat. No. 4,060,774 and Ser. No. 742,467 for "Electromechanical filter" dated 17th Nov., 1976 now U.S. Pat. No. 4,160,506.
It is known (see the article by R. A. JOHNSON, published in the report of the Symposium on Circuits and Systems Newton--April 1975, organized by the Institute of Electrical and Electronic Engineers) to create poles in the transmission characteristic by disposing auxiliary couplers between non adjacent resonators which will be termed "bridges" in the following text. The work which forms the subject of the aforesaid publication is summarized in the form of Table I.
Table I ______________________________________ Number of resonators between ends of Mechanical length Adjacent poles off the bridge of the bridge the pass-band ______________________________________ 2n 3.lambda./4 + p.lambda. 1 pole on each side of the band 2n .lambda./4 + p.lambda. phase corrector (2 conjugate complex poles) 2n -1 3.lambda./4 + p.lambda. 1 pole of frequency lower than the band 2n - 1 .lambda./4 + p.lambda. 1 pole of frequency higher than the band ______________________________________
Where
N is a positive integer other than zero, PA1 P is a positive integer or zero and PA1 .lambda. the wavelength at the central frequency of the filter in the mode of vibration chosen for the bridge.
In regard to the construction, it is important to minimize the length of the bridges in order to eliminate any danger of accident being introduced into the characteristic by a parasitic oscillation at a natural frequency of the said bridge. It is therefore desirable to limit the length of the bridge to one quarter or three quarters of the wavelength in the mode of vibration used (p=0 in Table I). In order to make the interconnection between non adjacent resonators, this length must be at least equal to that of the couplers between said resonators and, generally, even longer, because it is necessary to straddle a number of resonators. Therefore, there is incompatibility between the condition of minimum length of the bridge and the physical length necessary for the interconnection of non adjacent resonators.
There has been described in DOS No. 22 13 875, filed in the German Federal Republic on the 22nd Mar., 1972 by Licentia, a filter structure aimed at solving this problem, wherein the resonators are distributed in at least two separate planes, with their longitudinal axes parallel to a given direction. The bridge can then join two resonators belonging to two different planes. In all the constructional examples illustrating this patent application, the bridge straddles two intermediate resonators and no precise particulars are given with regard to the response curve of such a filter.